[tests] Print the exception tested with assertNoSite
[tests] Print the exception tested with assertNoSite

assertNoSite expects several exception when trying to create a
MWSite from url. But is is not clear, why creating the MWSite
fails. This can be happen if a urls doesn't exist any longer.
Therefore print the exception with this tests.

Change-Id: I80fcdfb68a15f75fe0650af83814c0e52e335e32

wikimedia/pywikibot-core,wikimedia/pywikibot-core